1. Field of the Invention
This invention relates to a system for detecting man made objects and natural features in sonar imaging and, more particularly, to a method for eliminating false target detections at the water column boundary in side scan sonar images.
2. Description of the Prior Art
Side scan sonars are used to create a sound reflectance image of the ocean bottom using short bursts of high frequency energy. A linear receiving array forms a narrow vertically oriented fan beam to receive the reflected energy. The delay time between the energy bursts or pings and the reflected energy sweeps a single line at a time of the image in range. The forward motion of the platform supporting the sonar lays down parallel raster lines of the image in the track direction.
In the water column region of the image, where it is too early for returns from the bottom, suspended objects image as highlights. In the near range zone at relatively high grazing angles, objects on the bottom image as highlights or highlights followed by short shadows. In the far range region, objects usually image as highlights followed by long shadows where the beam has been blocked from the ocean floor by the object.
Since the attenuation of the signal is as much as 70db between near and far range, modern sonars use a time varying gain (TVG) to compensate. In addition to this open loop compensation, a second form of gain control using feedback is applied by dividing the image into range segments and adjusting the gain within each range segment to yield an average level of intensity. This form of gain adjustment often yields undesirable anomalous effects such as shadows in the track direction after a bright object has been imaged.
Skilled sonar operators when using side scan sonar images to look for mines can compensate for most of these anomalous effects and readily distinguish the water column region from the ocean floor region of the image. When automatic detection methods are applied, however, preprocessing of the image is necessary to remove certain image anomalies before the detection process is run. In addition, certain image structures that degrade automatic detection algorithms must be eliminated.
Because of the high amount of noise and low resolution in sonar images, attempts to automate the mine detection process have been only moderately successful. In addition to an already difficult situation, anomalies introduced by the first stages of processing complicate the problem even more. Automatic detection processes employ some form of spatial differentiation of the image to detect highlight and shadow regions that are likely to correspond to mines as well as other things such as rocks. These detection methods often give false alarms at the boundary between the water column and the first return from the ocean floor. False alarms also occur whenever artifacts such as bands are introduced into the image due to unsophisticated normalization techniques. Accordingly, there is a need for an automated process that eliminates false detections at the water column boundary.